1. Field of the Invention
The present invention relates to a method for deciding a transmission priority of non-realtime data, and an apparatus and method for controlling inter-cell interference using the same; and, more particularly, to a method for deciding a transmission priority of non-realtime data, and an apparatus and method for controlling inter-cell interference using the same, which calculate uplink priorities of terminals in a corresponding cell using a user requested data transfer rate by each cell under a predetermined condition, provide services according to the calculated uplink priorities, and interrupt an uplink service in a corresponding cell based on resource utilization and inter-cell interference in order to improve system performance.
This work was supported by the IT R&D program of MIC/IITA [2005-S-404-23, “Research and development on 3G long-term evolution access system”].
2. Description of Related Art
The performance of a code division multiple access (CDMA) system depends on interferences. The interferences are classified into an inner cell interference generated between terminals that share one base station and inter-cell interference generated between cells that use the same frequency. Also, the interferences are classified according to a service type into downlink interference that influences one terminal because a plurality of base station transmit signals to a terminal, and uplink interference that influences one base station because a plurality of terminals transmit signals to a base station.
Since a downlink service is provided to single user in a cell at a predetermined time, the downlink service does not have inner cell interference. In this case, inter-cell interference is only the consideration.
Since an uplink service is simultaneously provided to a plurality of users in one cell at a predetermined time, it is required to control not only inter-cell interference but also inner cell interference.
Since users or terminals usually have bad wireless channel environment at a boundary of a cell, the users not only deteriorates throughput of a corresponding cell but also generates inter-cell interference to adjacent cells because the users are very close to the adjacent cells. In order to improve system performance, it is necessary to define constraints for the users. It is also necessary to control an entire cell having users crowded at a boundary thereof in order to control the inter-cell interference.
The inter-cell interference acts on a plurality of cells. Inter-cell interference coordinate is a technology for improving overall performance of a CDMA system by reducing inter-cell interference.
There have been many studies made for developing an inter-cell interference control scheme for a downlink service. For example, a frequency division scheme and a time division scheme where introduced. The frequency division scheme sets up a frequency reuse factor differently. The time division scheme sets up a service priority differently according to time division.
At first, the frequency division scheme will be described.
Since a frequency resource of a system is limited, the same frequency is repeatedly used with a predetermined pattern. The major factor of the frequency division scheme is how much frequency is used and how effectively frequency is used in order to reduce inner cell interference. Generally, a frequency is differently setup for a center area from that for a boundary of a cell.
The inter-cell interference is not a big problem at a center area of a cell. It is because the center area is far away from adjacent cells. Also, the inter-cell interference does not badly influence overall system performance because users at the center area have good link status. Therefore, the same frequency can be used in the center area of each cell. It is because inter-cell interference between center areas of adjacent cells can be ignorable although it is not 0.
On the contrary, the inter-cell interference seriously influences cell boundary areas and is a major factor that deteriorates system performance. Therefore, all of cells use different frequencies at the cell boundary areas. Also, the center area of a cell uses a frequency different from that used at the boundary area of the cell. That is, efficiency of using a frequency may be improved by reducing inter-cell interference and using only one frequency.
Hereinafter, the time division scheme will be described. In the time division scheme, a frequency band is differently setup by a service class, and the same service is not provided to cells using the same frequency at the same time. Although the time division scheme is an algorithm having high efficiency for utilization of a limited frequency, the time division scheme has shortcoming of difficulty to apply the time division scheme when the number of users abruptly increases or when demands for a predetermined service abruptly increase.
Also, the inter-cell interference can be controlled through power control as well as frequency control. In general, power is controlled in consideration of only interference between users in the same cell for scheduling. However, power may be controlled in consideration of how much users influence another cell by calculating the optimal power.
In a non-realtime data service, an average transfer rate for non-realtime data is more important than a burst transfer rate in a view of QoS. It means that users of the non-realtime service may not have any problem although a predetermined service is not provided at a predetermined time.
Using such fact, a method for improving overall system performance is developed. That is, overall system performance may be improved by instantly closing a cell that generates a lot of inter-cell interference and has low cell resource utilization, for example, interrupting a service for all users.
Hereinafter, a method for deciding a transmission priority of non-realtime data according to the related art will be described.
If a CDMA system includes M uplink service terminals, a link gain between a base station i and a terminal k is given as gik. The link gain denotes the transmission power of a transmitting side, which is reached at a receiving side. A signal receiving intensity of a base station for a terminal k is gikpk where pk denotes the transmission power of the terminal k. Each of elements B(k) of a vector B having M elements denotes a base station connected to a corresponding terminal.
The following Equation 1 shows a signal to interference ratio (SIR). The SIR is used as a reference of system evaluation in a CDMA system.
                              SIR          k                =                                            g                                                B                  ⁡                                      (                    k                    )                                                  ⁢                k                                      ⁢                          p              k                                                                          ∑                                                      j                    =                    1                                    ,                                      j                    ≠                    k                                                  M                            ⁢                                                          ⁢                                                g                                                            B                      ⁡                                              (                        k                        )                                                              ⁢                    j                                                  ⁢                                  p                  j                                                      +            η                                              Eq        .                                  ⁢        1            where η denotes thermal noise which generated in nature and cannot be controlled.
Each of terminals transmits data to a base station as much as Rk. Inter-cell interference is controlled at each of time slots.
The data rate may be calculated from the SIR, based on the following Equation 2.Data Rate=10×Bandwidth×Log(1+SIR)  Eq. 2
Meanwhile, a packet scheduler for transmitting non-realtime data uses a proportional fairness (PF) algorithm.
The PF algorithm was developed for a downlink non-realtime data service. The PF algorithm provides a downlink service to a user who had the lowest data rate for a predetermined past time period first although users request a high QoS service such as a high data rate. The downlink service is a service provided in a downlink, and the downlink service is provided to only one user at a predetermined time.
The PF algorithm was introduced to overcome a fairness problem. The fairness problem is arisen if a service is continuously provided a predetermined user having a good link status by a scheduler that only emphasizes system performance such as a data processing amount for a predetermined time period.
The PF algorithm can be also applied to an uplink service. A priority of a user to receive the uplink service is decided based on the following Equation 3.
                              1          k                =                              D            k            req                                D            i                                              Eq        .                                  ⁢        3            where Dkreq denotes a data rate that a predetermined user requests. Di denotes a previously allocated average data rate and is updated at every time slot, based on the following Equation 4.Dk(n+1)=(1−1/T)Dk(n)+rk/T  Eq. 4where T is a time period for an average value, and rk denotes a data rate allocated at a time n.
If a user is scheduled, a requested data rate is allocated. However, if not, its value becomes 0.
It is impossible to allocate a user requested data transfer rate using the method for deciding a priority according to the related art if a user requests a comparative high data transfer rate although the user is located at a cell boundary area and has a bad link status. Nevertheless, if the requested data transfer rate is allocated, an amount of interference increases in overall, thereby deteriorating system performance.